clang-tidy: fix some warnings

Resolve the following:
```
sys_info_param.cpp:12:11: style: Variable 'callback' can be declared as reference to const [constVariableReference]
    auto& callback = iterator->second;
          ^
include/ipmid/sessionhelper.hpp:49:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
include/ipmid/sessionhelper.hpp:36:14: style: The scope of the variable 'tempSessionHandle' can be reduced. [variableScope]
    uint16_t tempSessionHandle = 0;
             ^
include/ipmid/sessionhelper.hpp:77:47: performance: Function parameter 'objectPath' should be passed by const reference. [passedByValue]
bool isSessionObjectMatched(const std::string objectPath,
                                              ^
include/ipmid/sessionhelper.hpp:50:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
transport/serialbridge/test/serial_unittest.cpp:21:13: style: The scope of the variable 'c' can be reduced. [variableScope]
    uint8_t c;
            ^
```

Change-Id: I5a34fc50a95c33367e8b5d67dcb94c46b8def94a
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/sys_info_param.cpp b/sys_info_param.cpp
index 79bb29c..8cebb9c 100644
--- a/sys_info_param.cpp
+++ b/sys_info_param.cpp
@@ -9,8 +9,7 @@
         return std::make_tuple(false, "");
     }
 
-    auto& callback = iterator->second;
-    auto s = callback();
+    auto s = iterator->second();
     return std::make_tuple(true, s);
 }